Recently, the influence of a particulate matter emitted from a diesel engine mounted on truck or the like, on environment has been taken up seriously. As an important means for capturing and removing such a particulate matter, a honeycomb filter for exhaust gas purification is in use.
The honeycomb filter for exhaust gas purification generally has a structure possessing a plurality of through-holes which are separated from each other by porous partition walls and which are plugged alternately at its exhaust gas inlet face and its exhaust gas outlet face. The honeycomb filter captures and removes a particulate matter contained in an exhaust gas, passed through the partition walls in the filter from the open through-holes of the exhaust gas inlet face forcibly.
In such a honeycomb filter for exhaust gas purification, it is necessary to burn and remove, for filter regeneration, the particulate matter deposited in the through-holes (which is a flow path of exhaust gas) periodically or intermittently in order to secure the continuous use of filter.
This combustion of fine carbon particles, however, invites local heating (to high temperatures) of honeycomb filter and a difference in thermal expansion arises between sites of heating to high temperatures and other sites; as a result, thermal stress appears in the honeycomb filter and cracks, etc. are generated, making the continuous use of honeycomb filter very difficult, which is a problem.
As a stress-reducing means for elimination of the above problem, JP-A-59-199586 discloses a honeycomb structure having a large number of through-holes surrounded by partition walls, characterized in that through-holes having at least one slit in the partition walls surrounding the through-holes are provided substantially uniformly at given portions of the honeycomb structure.
In this honeycomb structure, small slits are uniformly distributed therein to increase the freedom of deformation and a reduction in thermal stress is obtained. However, since no consideration is made to the fact that thermal stress appears non-uniformly in various positions of honeycomb structure, the above means has been insufficient for stress reduction of a honeycomb structure used under severe environments where the non-uniformity of temperature distribution in honeycomb structure is larger.
Meanwhile, JP-A-8-28246 discloses a ceramic honeycomb filter of improved durability obtained by dividing a honeycomb structure into a plurality of honeycomb members for thermal stress relaxation and adhering the individual honeycomb members with an elastic sealing material obtained by bonding at least a three-dimensionally interwoven inorganic fiber and inorganic particles with each other via an inorganic binder and an organic binder.
In this honeycomb filter, however, since all the sides of each honeycomb member are adhered with a sealing material for increased durability, the thermal stress relaxation brought about by dividing the honeycomb structure into plurality of honeycomb members is reduced greatly and no sufficient thermal shock resistance has been shown in the continuous use over a long period.
The present invention has been made in view of the above problems and aims at providing a honeycomb filter for exhaust gas purification which has a high purification ability, yet possesses high thermal shock resistance, and can be used continuously over a long period.